For transferring information via a meshed network, path selection principles have so far been applied which have the common object of utilizing the total transfer capacity of the network, i.e. the avoidance as far as possible of traffic congestion. According to the principle described in the U.S. Pat. No. 3,111,559, each call connection is started by scanning the engagement condition of all the paths for the desired information transfer between two exchanges, and the unengaged path first discovered is used for the connection. With stored program controlled meshed network systems, path selection is controlled by centralized or decentralized path selecting units, which select, e.g. according to the U.S. Pat. No. 3,231,676, the most suitable path for the information transfer in question. The path selection process is here more complicated than the subsequent call connection process.
In telecommunication systems with a total traffic density which is seldom greater than the traffic capacity of a link between two exchanges, the very sophisticated path selection functions of a meshed network are avoided by arranging from the start an encircling loop, instead of the meshed network, all the exchanges in the system being connected to the loop. The calls are then set up with the aid of simple control functions, but traffic congestion is obtained when all the information transfer channels of the loop are already engaged. Loop systems are more vulnerable than meshed network systems. Arranging a single one-way encircling loop, the whole system will fail due to a single loop failure. Vulnerability decreases if spare loops are arranged, and decreases still further if the formation of a composite loop is enabled from fault-free parts of different original loops. Conventional loop systems are offered, e.g. by International Business Machines Corporation under the designation "IBM 8100 Information System", but users such as police, military services, airports, petro-chemical plants etc., requiring a telecommunication system which is as invulnerable as possible, have so far had to rely on conventional meshed networks with complicated control functions for path selection and call connection, although the information transfer capacity of a loop system would have been sufficient.